The central objective of this project is to determine the mechanisms regulating Cl- and K+ channels involved in active Cl- and K+ secretion across the mammalian colonic epithelium, since these channels are key elements in stimulating these distinct modes of ion secretion;Mucosal secretory responses are a balance between activator and repressor pathways. Secretory diarrhea can result from a pathophysiologic stimulation of crypt epithelial cells, which actively secrete Cl- and K+ in response to increased intracellular cAMP. Conditions such as inflammatory bowel disease and cystic fibrosis also lead to altered ion and water transport via actions at these secretory epithelial cells. Inappropriate inhibitions of repressor pathways, in particular, are a likely source of secretory diarrhea for patients with ulcerative colitis. The working model is that active secretory processes for Cl- and K+ occur in columnar cells of the colonic crypt epithelium, such that these cells are a locus of dysfunction in many gastrointestinal diseases. Ion channels involved in Cl- and K+ secretion will be examined using patch-clamp electrophysiologic techniques to record membrane ionic currents. Recordings from isolated crypts allow study of both apical and basolateral membrane channels as well as the second messengers controlling ion channel activity. Fluorescent probes will be used to monitor involvement of cell Ca++ and pH as second messengers controlling ion channels. The specific aims are to determine regulatory mechanisms for the basolateral membrane K+ channels, the Cl- basolateral membrane channels and the apical membrane K+ and Cl- channels involved in fluid secretion. The long-term objective is to determine the regulatory mechanisms acting on the multiple secretory functions of intestinal epithelial cells during physiologic stimulation. These distinct modes of ion channel control are the basis for excessive and impaired secretory functions that occur in secretory diarrhea, ulcerative colitis and cystic fibrosis. The close relationship between prostaglandin stimulation of secretion and inflammatory responses provides an opportunity to develop pharmaceutical agents that differentially target the prostanoid receptor subtypes involved. Differences in the ion channel types supporting each mode of secretion also can be exploited to develop further agents for selective clinical intervention, without producing deleterious side effects. Thus, this project will lead to greater insight into the regulatory control of colonic fluid secretion and likely to therapeutic interventions for patients with life-threatening secretory activity.